REPRODUCTION 



515 



fertilized ovum divides to form the first cells df the embryo: 

 such eggs are named holoblastic. 



The Maturation of the Ovum. From time to time, usually at 

 intervals of about four weeks, in a woman of child-bearing age, 

 certain ova after attaining the size and structure described in the 

 preceding paragraph undergo further changes by which the egg- 

 cell is rendered capable of fertilization. These phenomena, known 

 as the maturation of the ovum, result in separation of small parts of 

 the nucleus or germinal vesicle and cell protoplasm from the rest. 

 They are essentially typical cases of indirect cell division (p. 19). 

 The cell-body shrinks a little so as not quite to fill the zona pellu- 

 cida, and the germinal vesicle approaches one side. Meanwhile the 

 nuclear membrane and karyoplasm form the chromatic loop and 

 this divides into the usual two sets of Vs. One set of these, with 

 part of the nuclear plasm, now separates a little of the cell with 

 protoplasm to form a small cell, the first polar globule (c, Fig. 155). 

 The much larger cell resulting from the 

 division and representing the remainder 

 of the vitellus and nucleus now repeats 

 the process, and gives rise to the second 

 polar globule. In Fig. 155 the first 

 polar globule is shown at c, as already 

 separated, and the nucleus, d, is divid- 

 ing, preparatory to the formation of the 

 second one. The stage of karyokinesis 

 is more advanced than those represented Fir,. IBS. An ovum 



ii-i re to form the second polar glo- 



in Fig. 10. The two polar globules lie lor bule a< 20n a peiluoida; 6. 

 a time (Fig. 156) within the zona pcllu- g 

 cida in the space left by the shrinkage of 



the vitellus. but take no part 111 the preparatory to the separatum 



j ,!: of the second polar globule; 



formation of the embryo and soon clisap- t , vito n us> showing radial ar- 



pear. The rest of the original ovum is 



now mature and ready for fertilization; 



its nucleus is known as the female pronudes, fn, Fig. 156. 



passes towards the center of the ovum and forms the usual 



ticulum of karyoplasm found in normal resting nuclei 



Ovulation. From puberty, during the whole chil, 

 period of life, certain comparatively very large Graafian folh 

 may nearlv always bo found either close to the surface ot th, ovary 



